1. Field of the Invention
This invention relates to an in-line three beam type electron gun for color cathode ray tubes used for color cathode ray tubes which are components, for example, of color kinescopes and color display apparatus.
2. Description of Related Art
In general, the resolution characteristics of a color cathode ray tube depends greatly on size and configuration of an electron beam spot on the fluorescent screen. In detail, good resolution cannot be obtained unless the spot diameter of such electron beam is small and approximately completely round.
The path of an electron beam from an electron gun for a cathode ray tube to a fluorescent screen becomes longer when increasing in deflection angle of the electron beam. Therefore, a focus voltage is maintained so that an electron beam spot having a small diameter and complete round is obtained at the center of the fluorescent screen, such focus voltage results in an over focus at the peripheral portion. As a result, a beam spot having a small diameter and completely round can not be obtained around the peripheral portion, and good resolution can not be obtained.
To meet the recent requirement for wide deflection angle of an electron beam, the dynamic focus type electron gun for cathode ray tube, in which a high focus voltage is applied to an electron beam which impinges on the periphery of the fluorescent screen to suppress the main lens function has been developed. However, the dynamic focus type is not suitable for the in-line three beam type electron gun for cathode ray tube as it is. In detail, in the case that the deflection magnetic field of a deflection yoke is uniform in a conventional in-line three beam type electron gun for a cathode ray tube having three cathodes arranged on a horizontal straight line, though an electron beam is converged at the center of the fluorescent screen, but such uniform magnetic field leads to vertical-bow-shape convergence error (over convergence) on the top, bottom, right, and left periphery of a fluorescent screen as shown in FIG. 17. In FIG. 17, R (red) and B (blue) represent both side electron beams and G (green) represents the central electron beam. The same is true in the description hereinafter.
Therefore, heretofore the dynamic convergence has been performed with horizontal deflection magnetic field distribution by means of a deflection yoke in shape of a pin-cushion and with vertical deflection magnetic field distribution in shape of a barrel. However, in the case of a deflection yoke having such structure, an electron beam, which passes through a yoke and is deflected toward the periphery of the fluorescent screen, receives convergence action (convex lens effect) in the perpendicular direction (vertical direction) and, on the other hand, receives divergence action (concave lens effect) in the horizontal direction (parallel direction). As the result, an electron beam forms not a complete round spot but a horizontal elliptical spot on the periphery of the fluorescent screen. Therefore, a deformed spot of an electron beam and poor focusing performance on the periphery of the fluorescent screen are a problem.
To solve such problem, for example, Japanese Patent Laid-Open No. Sho 61-99249, Japanese Patent Laid-Open No. Sho 62-237642, and Japanese Patent Laid-Open No. Hei 3-93135 disclose electron guns for cathode ray tubes incorporated with a so-called electrostatic quadrupole lens (refer to simply as quadrupole lens hereinafter).
FIG. 13A is a conceptual diagram of an electron gun for color cathode ray tube having a built-in quadrupole lens which is used popularly. In this electron gun for color cathode ray tubes, a certain focus voltage V.sub.F is applied to the 5.sub.-1 electrode 51 through a stem. On the other hand, a superimposed voltage (V.sub.F +V.sub.DF) comprising a dynamic focus voltage V.sub.DF (refer to FIG. 8) which is synchronous with horizontal deflection of the focus voltage V.sub.F and the focus voltage V.sub.F is applied to the third electrode 13 and 5.sub.-2 electrode 52. Thereby a quadrupole lens is formed between the 5.sub.-2 electrode 52 and the sixth electrode 16, and the focus lens formed between the 5.sub.-2 electrode 52 and the sixth electrode 16 is varied in strength. As the result, the shape of an electron beam on the periphery in the right-left direction of the fluorescent screen is improved. On the face of the 5.sub.-1 electrode 51 which is facing to the 5.sub.-2 electrode 52, a plate 151 on which vertical electron beam apertures 151A, 151B, and 151C are formed as shown in FIG. 13B is provided. On the other hand, on the face of the 5.sub.-2 electrode 52 which is facing to the 5.sub.-1 electrode 51, a plate 152 on which horizontal electron beam apertures 152A, 152B, and 152C are formed as shown in FIG. 13C is provided.
Further, FIG. 13D is a conceptual diagram of an electron gun for color cathode ray tubes having a built-in quadrupole lens which is used popularly. In this electron gun for color cathode ray tubes, a certain focus voltage V.sub.F is applied to the 5.sub.-2 electrode 52 through a stem. On the other hand, a superimposed voltage (V.sub.F and V.sub.DF) comprising a dynamic focus voltage V.sub.DF (refer to FIG. 8) which is synchronous with horizontal deflection of the focus voltage V.sub.F and the focus voltage V.sub.F is applied to the third electrode 13, 5.sub.-1 electrode 51, and 5.sub.-3 electrode 53. Thereby quadrupole lenses which act inversely each other are formed between the 5.sub.-1 electrode 51 and 5.sub.-2 electrode 52 and between the 5.sub.-2 electrode 52 and 5.sub.-3 electrode 53, and a focus lens formed between the 5.sub.-3 electrode 53 and the focus lens formed between the 5.sub.-3 electrode 53 and the sixth electrode 16 is varied in strength. As the result, the shape of an electron beam on the periphery in the right-left direction of the fluorescent screen is more improved. On the plane of the 5.sub.-1 electrode 51 which is facing to the 5.sub.-2 electrode 52 and on the plane of the 5.sub.-2 electrode 52 which is facing to the 5.sub.-3 electrode 53, plates 151 on each of which vertically long electron beam apertures 151A, 151B, and 151C are formed as shown in FIG. 13B are provided. On the other hand, on the plane of the 5.sub.-2 electrode 52 which is facing to the 5.sub.-1 electrode 51 and on the plane of the 5.sub.-3 electrode 53 which is facing to the 5.sub.-2 electrode, plates 152 on each of which horizontally long electron beam apertures 152A, 152B, and 152C are formed as shown in FIG. 13C are provided.
By providing quadrupole lenses, as an electron beam approaches the end in the horizontal direction of a fluorescent screen, the electron beam receives divergent action (concave lens effect) in the perpendicular direction (vertical direction) and, on the other hand, receives convergent action (convex lens effect) in the horizontal direction (parallel direction). As the result, the electron beam forms a nearly complete round spot on the periphery of the fluorescent screen.
A quadrupole lens exhibits significant effect. In the prior art disclosed in the above-mentioned Japanese Patent Applications Laid-Open, the quadrupole is effective on three electron beams equally. However as shown in the conceptual diagram of FIG. 14, three electron beams which are emitted from an electron gun and impinge on the right and left periphery of a fluorescent screen are different each other in the position in a magnetic field of a deflection yoke 2 which these electron beams pass through. As the result, three electron beams receive convergent action and divergent action in the magnetic field of the deflection yoke differently in the magnitude of the actions. Therefore, three electron beams form spots distorted differently each other in extent on the right and left periphery of the fluorescent screen 4. In FIG. 14, the reference number 3 represents a glass bulb.
Usually the focus voltage is adjusted so that the center electron beam (G) out of three electron beams forms an electron beam spot of desired shape. In this case, when three electron beams impinge on the right side of the fluorescent screen 4, the electron beam R receives action of the deflection magnetic field formed by the deflection yoke 2 more intensely than the electron beam G and the electron beam B. As the result, the electron beam R forms a beam spot on the fluorescent screen 4 distorted more seriously than other electron beams. On the other hand, when three electron beams impinge on the left side of the fluorescent screen 4, the electron beam B receives action of the deflection magnetic field formed by the deflection yoke 2 more intensely than the electron beam G and the electron beam R. As the result, the electron beam B forms a beam spot on the fluorescent screen 4 distorted more seriously than other electron beams. Schematic electron beam spots on the fluorescent screen 4 are shown in FIGS. 15A and 15B. FIG. 15A shows the electron beam spots obtained when an electron gun for color cathode ray tubes having one set quadrupole lens structure shown in FIG. 13A is used. On the other hand, FIG. 15B shows the electron beam spots obtained when an electron gun for color cathode ray tubes having two set quadrupole lens structure shown in FIG. 13D is used. The electron beam spots obtained when an electron gun for color cathode ray tubes having two sets of quadrupole lens structures is more improved than the electron beam spots obtained when an electron gun for color cathode ray tubes having one set of quadrupole lens structures.
In some recent large size color display monitor having high resolution, red characters formed on the right side of the fluorescent screen 4 is unclear, and blue characters formed on the left side of the fluorescent screen 4 is unclear.
As one method to solve such problem, a method that the diameter of an electron beam is minimized at the center of a magnetic field of a deflection yoke 2 has been known. In detail, by minimizing the diameter of an electron beam at the center of a magnetic field of a deflection yoke, the effect of a magnetic field of a deflection yoke 2 on the electron beam dependent on the position through which the electron beam passes is suppressed as small as possible.
The diameter of a beam spot of an electron beam at the center of a fluorescent screen is calculated using the equation (1) described herein under. EQU Beam spot diameter={(M.times.d.sub.c +M.times.C.sub.s .times..theta..sup.3 /2).sup.2 +.DELTA.d.sub.rep.sup.2 }.sup.1/2 . . . (1)
Wherein M represents an image multiplication factor, d.sub.c represents an imaginary object spot diameter (diameter of cross-over), C.sub.s represents a spherical aberration coefficient, .theta. represents a divergent angle of an electron beam incident to the main focus lens, and .DELTA. d.sub.rep represents a diameter increment (repulsion) due to mutual repulsion of electrons. Assuming that a voltage of the third electrode 13 is V, the following equation (2) holds. EQU d.sub.c .theta..times.V.sup.1/2 =constant . . . (2)
FIG. 16 shows schematic and optical focusing of an electron beam emitted from an imaginary object point (X) on a fluorescent screen by means of a main focus lens formed between the fifth electrode and sixth electrode. The focusing represented by a solid line [A] in FIG. 16 represents the focusing when the minimized beam spot diameter of an electron beam in the equation (1) is obtained. The focusing represented by a dotted line [B] represents the focusing when the image multiplication factor M is low, that is, when the electron beam is emitted from an imaginary object point (Y) positioned farther than the imaginary object point (X). Further, the focusing represented by a dashed line [C] represents a focusing when the electron beam is focused with a smaller divergent angle .theta. than the divergent angle for minimizing the beam spot diameter of an electron beam in order to minimize the diameter of the electron beam at the center of the magnetic field of the deflection yoke 2.
In the focusing shown with the dashed line [C], because the diameter of the electron beam at the center of the magnetic field of the deflection yoke 2 is minimized, the adverse effect of the magnetic field of the deflection yoke 2 on the electron beam, which depends on the position through which the electron beam passes, is reduced. However, because the equation (2) holds between the imaginary object point diameter d.sub.c and divergent angle .theta., this focusing involves a problem that the beam spot diameter of an electron beam on the fluorescent screen 4 is larger than that of the focusing shown with a solid line [a].
For example, in a current 20 inch type color display monitor, the difference between the focus voltage that is required to obtain beam spots of electron beams R, G, and B shown in FIG. 15A and the focus voltage that is required to equalize the beam spot of the electron beam R on the right end of the fluorescent screen 4 to the beam spot of the electron beam G shown in FIG. 15A is as high as about 100 V. Naturally, as the shape of the beam spot of the electron beam R at the right end of the fluorescent screen 4 is equalized to the shape of the beam spot of the electron beam G, the shape of the beam spot of the electron beam G is degraded. Therefore, minimization of the electron beam diameter at the center of the deflection yoke 2 can not be effective as a method to solve the problem described above.
Accordingly, it is the object of the present invention to provide an in-line three beam type electron gun for color cathode ray tubes which is capable of equalizing shape of beam spots of the three electron beams at the right and left ends of a fluorescent screen.